Bacteriophage therapy
Antibiotics suck - they wind up creating resistant bacteria, and they're expensive to manufacture. The CDC has declared microbial resistance a threat to global health, predicting 10 million deaths by 2050. Phage therapy would instead use viruses that bind selectively to the cell receptors of bacteria to cause the bacteria to lyse. We've recently isolated phage-encoded lytic enzymes so there's a lot we could potentially do here.
Energy-efficient HVAC
Buildings are one of the largest consumers of world-wide energy, and we're still far from Carnot's theoretical limit when it comes to building efficient cooling systems. Geothermal and air-source heat pumps have fairly good CoP's, and you could get farther by separating out the humidity system from the cooling system. Billions of ACs will be sold in the next few decades (especially in India) so this goes a long way towards mitigating climate change.
Adoption
Many families in the US wait > 2 years to find a child to adopt. There are many fragmented adoption agencies, each with connections to local churches etc. that charge $40k-$50k per adoption (mostly for legal services). One could perhaps build a SaaS solution to help families find the best adoption plan (it's legal to have an adoption agency in State X, an attorney in State Y, etc.) and minimize legal costs through automation. Perhaps you could even build an adoption agency licensed in all 50 states that provides a really streamlined (but diligent & thoughtful) process for the whole thing.
Nuclear ships
90% of global transport is through container ships, and these typically take 2 weeks to cross the Atlantic. That's because they actually go slow (10-15 knots) to save money on oil (drag ~ v^2). As modular nuclear reactors (SMRs) become safer and standardized, one could in theory build nuclear ships that would be better for the environment and deliver packages faster. How would that impact the global supply chain?
Feel free to ping me if you're working on any of these or would like to chat about them! My email is first_name dot last_name at gmail.com.